1. Field of the Invention
The present invention relates to a semiconductor circuit generating a constant current with a small temperature dependence, preferably used as a reference current circuit or the like.
2. Description of the Related Art
Conventionally, constant current output insensitive to temperature environment, or temperature-independent current output, has generally been obtained by combining a circuit called “band gap reference circuit” with a voltage-current conversion circuit. The band gap reference circuit is a reference voltage circuit capable of generating a constant output voltage without temperature dependence. A constant output current can be obtained by converting the constant output voltage of the band gap reference circuit by a voltage-current conversion circuit.
FIG. 5 is a circuit diagram showing a configuration of a reference current circuit 50 configured using a band gap reference circuit and a voltage-current conversion circuit. The reference current circuit 50 is configured, as shown in FIG. 5, as having amplifiers 51, 53, pnp-type bipolar transistors Q51 to Q53, p-type MOS (metal oxide semiconductor) transistors M51 to M55, and resistors R51 to R53.
Bases and collectors of the transistors Q51 to Q53 are grounded (connected to the ground potential). An emitter of the transistor Q51 is connected to a drain of the transistor M51, and an emitter of the transistor Q52 is connected via a resistor R51 to a drain of the transistor M52. An emitter of the transistor Q53 is connected via a resistor R52 to a drain of the transistor M53.
Gates of the transistors M51 to M53 are commonly connected to the output end of the amplifier 51. Input ends of the amplifier 51 are connected respectively to an interconnection point of the emitter of the transistor Q51 and the drain of the transistor M51, and to an interconnection point of the resistor R51 and the drain of the transistor M52. Sources of the transistors M51 to M55 are connected to a power source circuit 52, from which power source voltage VCC is supplied.
A drain of the transistor M54 is grounded through the resistor R53. Gates of the transistors M54, M55 are commonly connected to the output end of the amplifier 53. Input ends of the amplifier 53 are connected respectively to an interconnection point of the resistor R52 and a drain of the transistor M53, and to an interconnection point of the resistor R53 and a drain of the transistor M54. A constant output current Iout is output from a drain of the transistor M55.
In FIG. 5, ratio of size of the transistor Q51 and transistor Q52 is set to 1:N (N>1), and ratio of size of the transistor M51 and transistor M52 is set to m:1 (m>1). Ratio of size of the resistor R51 and resistor R52 is set to 1:k (k>1). For example, the transistor Q52 can be realized by using N transistors having the same size with the transistor Q51, and the transistor M51 can be realized using m transistors having the same size with the transistor M52. Similarly, the resistor R52, for example, is realized by using k resistors having the same size with the resistor R51.
It is generally known that base-to-emitter voltage VBE of bipolar transistor has a negative temperature characteristic of approximately −2 mV/° C. Defining now base-to-emitter voltages of the transistors Q51, Q52 as VBE1 and VBE2, respectively, difference therebetween ΔVBE (=VBE1−VBE2) is known to show a positive temperature characteristic. As is obvious from FIG. 5, the interconnection point of the emitter of the transistor Q51 and the drain of the transistor M51, and the interconnection point of the resistor R51 and the drain of the transistor M52 have the same potential, so that the resistor R51 is exposed to potential difference ΔVBE, and current flowing through the resistor R51 also shows a positive temperature characteristic by contribution of the potential difference ΔVBE.
FIG. 5 therefore teaches that a proper selection of a value of k so as to equalize temperature-dependent amounts of changes (absolute values) in the base-to-emitter voltage VBE of the transistor Q53 and in (ΔVBE×k) at the resistor R52 (or so as to cancel the temperature-dependent influences) makes it possible to obtain an output voltage of approximately 1.2 V in a temperature-independent manner. Successive conversion of the constant output voltage without temperature dependence by a voltage-current conversion circuit, which comprises the amplifier 53, transistors M54, M55 and the resistor R53, results in output of a constant output current Iout.
In this configuration of the circuit, based on use of the band gap reference circuit, intended for obtaining a constant output current with a small temperature dependence, it is necessary to additionally provide a voltage-current conversion circuit, as described in the above, in order to obtain a constant output current, because use of a general band gap reference circuit can only provide a circuit generating a constant output voltage.
A proposal has been made also on a band gap reference circuit as typically disclosed in Patent Document 1, operable at a low power source voltage. The circuit configured to generate a constant output voltage and to convert it into a constant output current, however, raises a difficulty in lowering the power source voltage, because elimination of the temperature dependence needs an output voltage of at least as high as approximately 1.2 V due to various physical conditions.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2000-323939